Apparatus for post-tensioning prestressed concrete



W. F. KELLY sept. 3, 1968 APPARATUS FOR POST-TENSIONING PRESTRFISSED CONCRETE 2 Sheets-Sheet 1 Filed April 11, 1967 mammal I NVE NTOR W/'///`am F Kel/y W. F. KELLY sept. 3, 196s APPARATUS FOR POST-TENSIONING PRESTRESSED CONCRETE Filed April l1, 1967 2 Sheets-Sheet 2 RY on Te mw mm .m w w ATTORNEYS' United States Patent O ABSTRACT oF THE DISCLOSURE The post-tensioning of prestressed concrete by means of forms between which concrete is to be poured, a dead end anchor secured to one form and a live end anchor secured to an opposite form, each anchor including `a tapered case, a jaw assembly within the case for engagement with a tensioning member extending therebetween, a helix circumjacent the outer periphery of the tapered case to effect stress distribution, the cases adjacent forms being provided with fastening members to facilitate securement to the forms, means engaged with the tensioning member adjacent the outer face of the live anchor form for prestressing the tensioning member, and spring means interposed between the prestressing means and form for permitting a limited movement of the tensioning member between the forms when a predetermined force is exerted thereon.

This invention is directed to apparatus for post-tensioning prestressed concrete, and is a continuation-in-part of application Ser. No. 490,421, filed Sept. 27, 1965.

OBJECTS AND DESCRIPTION OF An object of this invention is to provide a series of spaced tensioning assemblies, each assembly including a dead end anchor unit, secured to one end of the concrete form, and ra live end anchor unit secured to an opposed form, the anchors being equipped with appurtenances for supporting and holding the terminals of a high-strength tensioning member or tendon, which is stressed to a predetermined tautness before pouring the concrete or .other material, means being provided for permitting limited movement of the tensioningmember between -the forms upon exertion of a predetermined force thereon, the appurtenances also including means positioned near each anchor and adjustably connected to the opposite ends of the member, the means including,v interalia reaction plates embedded in the earth and supported-by reaction stakes to complete the force couple through the earth.

Another object of the invention is to provide apparatus of the character described wherein the means for permitting limited -movement of the tensioning member embodies a convolute spring around the tensioning member between the live anchor form and the means for exerting the tautness on the tensioning member, the spring being compressed when a predetermined force is exerted on the tensioning member ybetween the forms, resulting in a limited giving of the tensioning member.

Another object of the present invention is to provide dead end and live end anchors, both of which include a tapered case, in which is a jaw assembly for engaging the tensioning member, and a-helix locatedfcircumjacent the outer periphery of the tapered case to effect stress distribution.

A further object is to provide an anchor, as described, with the addition of a fastening member coiled about the tapered case, the terminals of the fasteningrnember being formed to provide loops adapted for the reception of fastening means for securing the anchors to forms.

Other objects of the invention will be manifest from Patented Sept. 3, 1968 lCC the following description of the present preferred forms of the invention, taken in connection with the accompanying drawings, wherein:

FIGURE 1 is a fragmentary side elevational View illustrating the apparatus of the present invention installed between forms and illustrating the tensioning member in prestressed condition;

FIGURE 2 isl an enlarged plan view of Detail A of FIGURE l;

FIGURE 3 is a longitudinal sectional view, illustrating to advantage a portion of the jaw assembly forming a part of the anchor of the present invention;

FIGURE 4 is `a transverse sectional view of an anchor with the helix removed;

FIGURE 5 is an enlarged top plan view of the apparatus of the present invention illustrating to advantage the construction of the case helix and fastening loop, and the manner of securing the anchor to a form;

FIGURE 6 is an end elevational view of the anchor of the present invention per se;

FIGURE 7 is an elevational view of a modified form of anchor made in accordance with the present invention, and

FIGURE 8 is an end elevational view of the form 0f the present invention illustrated in FIGURE 7.

The apparatus of the present invention is illustrated for post-tensioning a prestressed poured-on-grade concrete slab, but is susceptible of use for any purpose to which an lanchor for post-tensioning prestressed concrete or other suitable material can be applied.

In FIGURE l, a pair of anchors are illustrated, the dead end anchor unit being designated 10 and the live end anchor unit being designated 12, both units being of the same Vbasic construction as set out in copending application Ser. No. 490,421.

As shown to advantage in FIGURES 3 to 6, each an- 'chor includes a tapered case 14, the termina-l portion of a large end of which is ilarer outwardly to provide an annular peripheral recess 16. A jaw assembly 18 of the type illustrated and described in application Ser. No. 490,421 is positioned in tapered case 14, which jaw assembly is shown to advantage in FIGURES 3 and 4. I aw assembly 18 includes jaw segments 20 having teeth 22 for engagement with tensioning member or tendon 24. law segments Z0 are preferably joined together by a resilient band 26.

In accordance with the objects of the present invention, a helix generally designated 28 is positioned circumjacent tapered case 14 and is substantially coextensive therewith. It will be noted from a consideration of FIG- URE 5 that the intermediate portion of helix 28, indicated at 30 is in substantially spaced relationship to the outer periphery of tapered case 14. One terminal of the helix, however, is reduced, as indicated at 32, for contiguous engagement with the enlarged portion of tapered case 14, the extreme terminal being adapted to be received by annular peripheral recess 16; the opposite terminal of helix 28 is also reduced as indicated at 34 for positioning proximate the nose of case 14. This maintains alignment of the helix on the approximate axis of the case and tensioning member.

In order to facilitate attachment of the anchor to a form, there is provided a fastening member generally designated 36 which is `adapted for attachment to tapered case 14 at a point near the terminal of the larger end thereof. Fastening member 36 includes a central portion 38 which is coiled l`around and in contiguous engagement with the outer periphery of case 14. The terminals of fastening member 36 are extended outwardly at 40 beyond the limits intermediate coiled section 30 of helix 28, and issue into loops 42 having apertures 44, as shown in FIGURES 5 and 6.

Anchors and 12 and fastening member 24 are adapted to be positioned Ibetween forms F, anchors 10 `and 12 being secured to the forms by nails 46 or the like which extend through apertures 44 of fastening member 36, as shown in FIGURES 1 and 5. The apparatus of the present invention is arranged in a similar manner to that shown and described in connection with application Ser. No. 490,421. Referring to FIGURE 1, there is illustrated the apparatus of the present invention and appurtenances for a poured-on-grade concrete slab, wherein a visqueen membrane 48 or the like is spread over compacted till 50. Conventional chairs 52 are positioned intermediate forms F for supporting engagement with tensioning member 24.

Forms F are held in place by anchoring stakes 54 which are driven into the ground. Dead end anchor 10 is adjustably connected to reaction means 56 comprising a steel reaction stake 58 and a steel reaction plate 60, the latter being completely embedded in the ground G and secured to stake 58 which is driven into the ground at an angle, as shown in FIGURE 1. Adjustment of tension on dead end anchor 10 is effected by a turn'buckle 62 threadedly connected to rods 64 and 66. A -free terminal of rod 64 is secured to ya hook 68 and the free terminal of rod 66 is engaged with a Abent portion of a nail 46 used to secure anchor 10 to form F.

Form F holding live end anchor 12 is in turn supported by a stake 70 driven into ground G, to which reaction means 72 are connected. Reaction means 72 includes a steel stake 74 engaged with Ia reaction plate 76 which is completely embedded in the ground. Adjustment of tension is effected by a turnbuckle 78 threadedly connected to rods 80 and 82. The free terminal of rod 80 is secured to a hook 84 fixed to stake 70 and the free terminal Vof rod 82 is engaged with a hook 86 carried by stake 74.

In accordance =with one of the objects of the present invention, there is provided means for lightly pretensioning member 27. This means includes a pretension anchor 88 which may be of conventional type, such as the Strandvise anchor manufactured by Reliable Electric Company. It is a salient feature of the present invention to provide, in association with pretensioning anchor 88, a prestressing spring 90 which, as shown in FIGURE 2, is interposed `between anchor 88 and form F. Pretensioning spring 90 is preferably a standard die-set type spring with a one half inch hole for the reception of a three-eighths inch tensioning member, a spring with Ka proportionately larger size hole -being employed for tensioning members of larger diameter. Optimum results have been obtained with a pretensioning spring in the blue strength range which in the three-eighths inch size produces a compressed loading of approximately two hundred -fty pounds for a threequarters inch spring closure which has been found to be adequate for the purposes of the present invention.

OPERATION In use of the present invention a plurality of anchors and tensioning members are preferably arranged in spaced parallel relation in the manner illustrated in FIGURE 1. After this has been done, tensioning member 24 is drawn taut through live anchor 12 until the desired tautness has been effected. Pretension anchor is then set to maintain the desired tension of member 24. In the event that undue stress is exerted on tension memhber 24 between forms F, such as by persons walking thereon or placing a weighty object on the member, prestressing spring 90 is compressed under the force, resulting in slight movement of pretensioning anchor 88 in the direction of form F and resultant giving of tensioning member 24 under the weight applied thereto. Upon removal of the weight, pretensioning spring 90 exerts Ia force on pretensioning anchor 88 to return the latter to its initial position, with a corresponding movement of tensioning member 24 between forms F. In this way, damage to the forms and connected parts is obviated.

When concrete or other material is poured between 4 forms F, members 24 maintain the position shown in FIGURE l spaced from membrane 48.

Helices 28 of anchors 10 and 112 act to lend greater strength and flexilibity than has heretofore been possible with better calculated stressed distribution, resulting in a superior anchor.

After the concrete has been poured and permitted to harden, forms F is removed, following which member 24 is post-tensioned with jacking equipment of the type disclosed in application Ser. No. 560,463 led by me June 27, 1966. 'Ihe terminals of nails 46 -beyond the concrete may be cut off or removed in any other suitable fashion.

EMBODIMENT OF FIGURlES 7 AND 8 In FIGURES 7 and 8 there is illustrated a modied form of the anchors of the present invention, which is similar to anchors 10 and 12 illustrated in FIGURES 1 through 6. Accordingly, like portions thereof are identified by like, primed numbers. It will be noted, however, that in this form of the invention no fastening member corresponding to member 36 of the form of invention illustrated in FIGURES l through 6 is employed. This embodiment of the invention is used when multiple an chors are ganged together intermediate the concrete forms where no fastening means is necessary, or are Ibuilt to fit special conditions.

While preferred embodiments of the present invention have been shown and described, it is nevertheless to *be understood that various changes may be made therein without departing from the spirit and scope of the claims hereto appended.

I claim:

1. An assembly for use in the post-tensioning of prestressed concrete comprising (a) spaced building forms between lwhich concrete is to be poured,

(b) a dead end anchor adjacent one of said forms,

(c) a live end anchor adjacent the other of said forms in opposed relation to said dead end anchor,

(d) means securing said anchors to the opposed faces of said forms,

(e) a tensioning member xedly held at one end to said dead end anchor by jaw segments,

(f) the opposite end of said tensioning member being stretched through and beyond said live anchor containing jaW segments,

(g) a jaw assembly retaining means-adjacent the external face of the live anchor form holding said tensioning member under predetermined tension, and

(h) means interposed between said retaining means and the live anchor form permitting limited movement of said tensioning member between said forms upon application of a predetermined force thereto.

2. The assembly of claim 1, wherein (a) said means interposed between said retaining means and the live anchor form is a convolute spnng.

3. The assembly of claim 1, with t-he addition of (a) at least one additional anchor engaged with said tensioning member immediate said forms.

4. 'I'he assembly of claim 1, wherein said live and dead end anchors each comprise (a) a tapered case,

(b) tensioning member engaging elements vwithin said tapered case, and

(c) a helix engaged with an extending circumjacent, the outer periphery of said tapered case to effect stress distribution.

S. The assembly of claim 4, with the addition of (a) a fastening member around the outer periphery of the live and dead end cases, said fastening member including (b) means adapted for the reception of a fastening element for securing the fastening member and con nected anchor to the form.

6. An anchor for use in the post-tensioning of prestressed concrete comprising (a) a tapered case,

(b) a jaw assembly slidably mounted in the case,

(c) said jaw assembly including jaw segments having teeth for gripping engagement with a tensioning member passed therebetween,

(d) a helical spring Ipositioned circumjacent the outer periphery of said tapered case to elect stress distribution,

(e) said helical spring being substantially coextensive wit-h the length of said tapered case, and

(f) a fastening member coiled around said tapered case,

(g) the terminals of said fastening member issuing into loops adapted for the reception of fastening elements.

References Cited UNITED STATES PATENTS 10/1885 Webb 2A-136 11/ 1952 Freyssinet 52-223 6/ 1964 yRhodes 52-230 1/ 1965 Ziolokowski 24--126 11/1965 Sweeney 24123.1 5/1966 Jost 24-126 8/1966 Carlson 52-573 12/ 1966 Rice 52--223 FOREIGN PATENTS 3/1956- Austria.

HENRY C. SUTHERLAND, Primary Examiner.

I. L. RIDGILL, JR., Assistant Examiner. 

